If your question is whether instant dry yeast is interchangeable with active dry yeast or Rapid-Rise yeast, the answer is yes provided that you use the proper amounts. In other words, one teaspoon of instant dry yeast cannot be substituted for one teaspoon of active dry yeast or one teaspoon of Rapid-Rise yeast and get identical results. While all three forms of yeast operate pretty much the same from the standpoint of chemistry, they are still different--from particle size to strain of yeast to rate of fermentation. You didn't mention wet yeast, such as fresh compressed cake yeast (like you see in small square packages with silver foil in the refrigerator section of the supermarket), but the active dry yeast and instant dry yeast forms can be substituted for that form of yeast also. That is why you will often see the following equivalencies:

Rapid-Rise yeast is significantly different from the other forms of yeast set forth above. It operates much faster (by around 50%), but its yeast cells also die faster. This makes the Rapid-Rise yeast most suitable for situations where a fast rise time is desired or needed. That isn't necessarily good for pizza doughs. Pizza doughs benefit far more from a flavor and quality standpoint from long, slow rises and, even better, fairly prolonged periods of retardation (refrigeration). If you use the Rapid-Rise yeast for a pizza dough, you will get an amazingly fast rise, but the crust resulting from the dough will taste like cardboard. In fact, I would choose real cardboard over a crust using Rapid-Rise yeast. Some time ago, I made a pizza using 2 packages of the Rapid-Rise yeast for about 4 cups of flour. The dough was ready for shaping and dressing in 20 minutes and, with preheating of the oven from the start, the pizzas were done well within an hour. By contrast, I have made pizza doughs of very high quality by just about every measure using as little as 1/8-1/4 teaspoon yeast (instant yeast), but with long, slow rise times and a long period of refrigeration.

But if you insist on knowing the relationships of Rapid-Rise yeast--which I long ago abandoned--to the other forms of yeast, the last time I saw the equivalencies they were as follows:

Keep in mind also that the different forms of yeast have different requirements. The compressed yeast cake and active dry yeast forms require "proofing", or "rehydration", in water or some other liquid. The instant dry yeast and Rapid-Rise yeast forms don't require such proofing. They can simply be combined with the other dry dough ingredients. That is one of the reasons I have personally gone almost entirely to instant dry yeast, such as was depicted in one of the earlier posts on this subject. It's cheap and if you freeze it in an airtight container it will last for well over a year if not much longer (note that the expiration dates used by yeast producers are for commercial bakeries only, where yeast is left open and exposed to high temperatures daily).

I believe Steve has it right. As best I can tell from some Internet searching this morning, Rapid-Rise is Fleischmann's brand of instant dry yeast . To be certain of that, however, I have sent an email to Fleischmann's to get confirmation one way or another (their site says I should get a response within 3 days). In the meantime, I also tried calling Red Star about its brand of instant dry yeast, where the best I could do was to leave a voicemail message, and I also tried sending emails to SAF and Red Star, using their websites, only to see the attempted emails "crash". The only site of the 3 major yeast producers that I called that allows the possibility of talking to a person is Red Star (which is part of SAF). It seems no one wants to make it easy for consumers to actually talk to a live human being about anything anymore. Whatever information or replies I get I will post.

As far as use of instant dry yeast is concerned, I still don't advocate excessive use, for the reasons I mentioned from my experience using 2 packets of the Rapid-Rise yeast in a single dough recipe. I should have faulted the recipe rather than the yeast. My apologies.

In a recent discussion with Pete-zza, I was reminded of a point regarding changes that have evolved by manufacturers, explaining why recommendations are made in many occassions not to proof Active Dry yeast with water prior to use, a technique that I have successfully applied with Red Star at home and witnessed professionally. I also noticed that a friend of mine who makes bread in a bread machine initially keeps the active dry yeast away from water, first pouring in her liquids, followed by flour, then active dry yeast.

I received a reply from Fleischmann's to the email I sent earlier today inquiring as to the similarities or differences between the Fleischmann's Rapid-Rise yeast and the instant dry yeasts of others (such as SAF/Red Star)--in the context of pizza doughs. Fleischmann's declined to comment on its competitors' instant dry yeast offerings, claiming not to have information on those products.

However, in the reply, Fleischmann's recommends using its Rapid-Rise yeast in a "One-Rise Method", described as follows (in quotes):

"One-rise method1.Set aside 1 cup of flour from the total amount (save for later use in the recipe). Mix remaining flour(s), RapidRise Yeast and all other dry ingredients in a large bowl.

2. Heat fats and all liquids except eggs until very warm (120 to 130F).

3. Stir very warm liquids into dry mixture. Mix in eggs if required. Mix in just enough reserved flour to make dough or batter.

5. Shape dough and place in prepared pan(s) as directed in recipe. Cover dough and set dough in a warm (80 to 85F), draft-free place, and let rise until doubled in size.

6. Bake as directed. Remove from pan and cool on wire rack."

When I revisited the pizza dough recipe I had used that was based on the Rapid-Rise yeast, it too used what might be called a One-Rise Method, except that the pizza dough was shaped, etc., and baked very shortly thereafter, with the finished product out of the oven within a half hour from beginning to end. (I even made a faster version, where I jacked up all the temperatures, and the pizza was out of the oven in about 20 minutes.)

Wondering whether there were other uses of the Rapid-Rise yeast, I went to the Fleischmann's website, at http://www.breadworld.com, where recipes are offered. I did a search for pizzas that turned up 29 for "Conventional Oven" and 16 for "Bread Machine". All of the Conventional Oven pizza recipes for conventional pizza recipes (there were a few fruit based recipes and other variants) call for the Rapid-Rise One-Rise Method in one form or another (depending on the ingredient variations of the specific pizza recipes), with processing (shaping, etc.) and baking taking place fairly shortly after a 10-15 minute or so rise, much like what I did in using my Rapid-Rise dough recipe. All of the conventional Bread Machine recipes call for use of Fleischmann's "Bread Machine" yeast, not the Rapid-Rise yeast.

I also went to the SAF and Red Star websites to check out their pizza recipes, to see if the same techniques were advocated. I could find only one pizza recipe at SAF, which uses their instant dry yeast and a slightly longer initial rise. Red Star has only a few conventional pizza recipes, using its "Quick-Rise" yeast which, I assume, is its version of the Rapid-Rise yeast. I should know more about this if and when I hear back from Red Star.

What surprises me the most is that the major yeast producers don't more aggressively advocate use of their instant dry yeasts for longer, multiple-rise doughs, including the possibility of refrigeration. I can only surmise that they have their active dry yeasts for those situations, and leave the Rapid-Rise yeast primarily for those applications where there is a need or desire to produce a finished product in a short period of time, rather than one of high quality.

Rapid Rise traditionally was for just that... occassions where you need dough quickly. Manufacturers also state that their instant yeast is far more active than Active Dry Yeast.

As manufacturers continue to plug away with yeasts, proofing is becoming less of a distinguishing factor for Active Yeasts. Whole Foods, for example, carries an organic "Active Yeast" that clearly states no pre-mixing with water is required. My Red Star Active Yeast gives instructions to add to flour or to water.

I have been going back and forth recently with Fleischmann's about their Rapid-Rise yeast--trying to learn what it is in relation to its instant dry yeast sold in one-pound bags at places like Sam's, Amazon, The Baker's Catalogue and elsewhere, and what it is in relation to similar products sold by its competitors. Getting direct answers to these questions has been like pulling teeth. I was told that "Instant yeast is fast rising and can be used as a substitute for Rapid Rise or Bread Machine Yeast in equal amounts" and "Fleischmanns RapidRise and Instant yeast when used the tradional [sic] method should be used the One-Rise Method", but they would not tell me what the true relationship is between the Rapid-Rise yeast and the Fleischmann's instant dry yeast sold in the one-pound bags. As a final try, I asked point blank whether the Rapid-Rise yeast was identical to, nearly identical to, or different in some way from their Fleischmann's instant dry yeast sold in the one-pound bags. The answer that came this morning is: "It is not the same, Instant Yeast is Industrial Yeast, but they are both fast-rising yeast and can be used interchangeable [sic]."

I have read that Fleischmann's markets differently to the consumer and industrial sectors. I don't think they want ordinary consumers to buy the bags of the instant dry yeast when they can make a great deal more money selling the small Rapid-Rise yeast packets or small bottles. And for the consumer market, it looks like Fleischmann's pushes the Rapid-Rise yeast and the One-Rise method, when it has applications that appear to go far beyond that one application. I don't get the impression at all that Fleischmann's wants people to know that.

I do get the impression though that they both share a common attribtue-- they are faster rising, and this needs to be considered when comparing its uses to active yeast, which does not require proofing either with various manufacturers these days.

That is essentially so but you can control the amount of instant dry yeast you use to slow down the fermentation process. Most of the pizza recipes for consumers at Fleischmann's consumer website call for using a lot of Rapid-Rise yeast in their One-Rise method. This allows one to make a pizza within about an hour in most cases, and sometimes even sooner. The last time I tried the Rapid-Rise yeast, the pizza was done well under an hour (and even less in one test where I boosted all the temperatures). The quality of the crust was poor because everything that took place within the dough was over in minutes, so there was no real fermentation and flavor-enhancing by-products. But if you use only a small amount of the Rapid-Rise yeast, like the 1/8 teaspoon I use in my New York style pizza dough, you should be able to get fermentation over a period of several days, just as with an active dry yeast. I haven't actually used the Rapid-Rise yeast that way--since I didn't know it could be so used--but that is what I would expect since, as best I can tell, it isn't materially different from a functionality standpoint from the regular instant dry yeast--like Fleischmann's sells in the one-pound bag at Sam's and elsewhere.

Today, I received a response from Red Star about its Quick-Rise yeast, which appears to be Red Star's competitive offering to the Fleischmann's Rapid-Rise yeast (although Red Star wouldn't admit it), and SAF's instant dry yeast. Red Star doesn't itself offer an instant dry yeast (it does have a bread machine yeast) but its parent company, SAF, does--including in a one-pound bag (like the SAF Red). I asked whether the Quick-Rise yeast and the SAF instant dry yeasts were the same. The answer I got is that they are so close that you wouldn't be able to tell the difference.

In light of what I have learned about yeast over the past few days, my best advice for someone who does a lot of pizza (or bread) baking is to find a good brand of yeast, whether it is active dry yeast or instant dry yeast, in one of those one-pound bags, since that will be the most cost effective. And then either follow instructions as to use or play around with the yeasts in your recipes until you get to understand how the yeasts work for you and the amounts needed to get the desired results. I don't think the particular brand matters all that much since you aren't likely to be able to tell the difference in the finished product.

That's sound advice. Unfortunately, too often I see people (especially newbies) presume after reading the discussion groups that instant is required for a good pizza, and that just isn't the case even with pros.